The invention relates generally to the field of power over local area networks, particularly Ethernet based networks, and more particularly to detecting the end of a start up phase.
The growth of local and wide area networks based on Ethernet technology has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The structure cable is also known herein as communication cabling and typically comprises four twisted wire pairs. In certain networks only two twisted wire pairs are used for communication, with the other set of two twisted wire pairs being known as spare pairs. In other networks all four twisted wire pairs are used for communication. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to; reduced cost of installation; centralized power and power back-up; and centralized security and management.
Several patents addressed to the issue of supplying power over an Ethernet based network exist including: U.S. Pat. No. 6,473,608 issued to Lehr et al.; and U.S. Pat. No. 6,643,566 issued to Lehr et al.; the contents of all of which are incorporated herein by reference.
The IEEE 802.3af-2003 standard, whose contents are incorporated herein by reference, is addressed to powering remote devices over an Ethernet based network. The above standard is limited to a powered device (PD) having a maximum power requirement during operation of 12.95 watts. Power can be delivered to the PD either directly from the switch/hub known as an endpoint power sourcing equipment (PSE) or alternatively via a midspan PSE. In either case power is delivered over a set of two twisted wire pairs.
The above mentioned standard defines certain electrical requirements applicable to normal powering, i.e. steady state operation, and separate limits for a start up phase. In particular, at Table 33-5 of the above standard, an output current limit during the startup phase, denoted IInrush, is defined separately from a maximum output current in normal powering mode, denoted Iport—max. The startup phase is characterized by the initial charging of an input capacitor of the PD, which presents an effective short circuit across the PSE at startup. At the end of the startup phase, the input capacitor of the PD is effectively charged.
A PD according to the above mentioned specification exhibits certain characteristics including polarity independence, a detection resistance, an optional classification functionality, a minimum input capacitance, and a minimum turn on voltage. In the event of a PD input capacitance being greater than or equal to 180 μF, the PD is further operative to control the inrush current during startup mode so as to ensure a maximum Iinrush of 400 mA. After detection and optional classification, the PD is not to close an isolating switch placing the input capacitance and load across the PSE until a minimum turn on voltage is detected by the PD.
The IEEE 802.3 at DTE Power Enhancements Task Force is in the process of developing a standard exhibiting higher power limits than the above IEEE 802.3af-2003 standard. It is anticipated that an increased operating power will be supported over wire pairs during normal powering mode, i.e. once the startup phase has completed. It is further anticipated that little, if any change, is to be made to the current limits of the startup phase, i.e. IInrush, which is set at limits significantly lower than the anticipated normal powering mode current. Thus, before a PD under the IEEE 802.3 at proposed standard can draw increased current, it must first ensure that the startup phase has been completed, and must further ensure that the PSE is aware of the end of the startup phase.
For example, in the event that the PD determines that the startup phase has completed while the PSE is still in the startup phase, the PD will attempt to draw increased current, consonant with the normal powering mode, while the PSE exhibits current limit IInrush. Such a condition will prevent proper operation of the PD, and may further result in increased stress at the PSE. A simple solution is to require a timer at the PD, which prevents the end of the startup phase from being determined before the expiration of a predetermined time period. A similar timer, with a slightly shorter predetermined time period, at the PSE would thus ensure that normal powering mode is achieved without being subject to IInrush. Unfortunately, such a requirement adds cost, requiring timers at both the PD and PSE.
What is needed, and not supplied by the prior art, is a means for determining, at the PSE, the end of the startup phase occurring at the PD.